I'll answer you the best I can here, though you might want to ask in TAW where I can guarantee you will receive the correct answers.
Renouncing Catholicism simply means returning to Orthodoxy. The two are what you would call separate "denominations," basically, though we definitely would not consider ourselves such (what with that whole "one holy catholic and apostolic Church" thing

). There are a few theological points that we feel would need fixing, basically.
What's un-Orthodox about Roman Catholicism? Well, as stated earlier, the main issues that caused the split were the papacy (the bishop of Rome was always the first among equals, not what he is now) and the insertion of the filioque - Orthodoxy condemns the idea that the Holy Spirit proceeds from the Son as He does from the Father as heretical. There are plenty of others that have arisen over the years, such as the Immaculate Conception of Mary.
We do not have a pope as do the Roman Catholics. There are a number of autocephalous churches (such as the church of Greece, Antioch, Russia, Bulgaria, Jerusalem, and so forth) and all have patriarchs, but none of them alone are infallible as the RCs claim the pope to be.
Somebody correct me if any of that is inaccurate. Again, visit TAW if you want some really good answers to your questions.